A double backplate microphone in MEMS device technology comprises a top backplate electrode and a bottom backplate electrode being arranged in parallel to each other, and a membrane being disposed between the top backplate electrode and bottom backplate electrode in parallel. The top backplate electrode, bottom backplate electrode and membrane are supported by a support structure. This arrangement is supported by a substrate.
In order to transmit sound pressure waves, for example speech, to the membrane interposed between the top backplate electrode and bottom backplate electrode, these electrodes may be perforated. Sound pressure waves cause the membrane to vibrate due to a pressure difference over both planes of the membrane. Hence, the air gap between the membrane and each of the backplate electrodes varies. The backplate electrodes and the membrane may comprise electrically conductive material(s). The variation of the membrane in relation to the backplate electrodes causes variation in the capacitances between the membrane and the bottom backplate electrode as well as between the membrane and the top backplate electrode. This variation in the capacitances is transformed into an output signal responsive to the movement of the membrane. The membrane may be biased by a bias voltage relative to the bottom backplate electrode and the top backplate electrode.
The double backplate microphone as described above schematically, suffers from parasitic capacitances, created inside the support structure. A first parasitic capacitance can be created inside the support structure between the membrane and the top backplate electrode. A second parasitic capacitance can be created inside the support structure between the membrane and the bottom backplate electrode. A third parasitic capacitance can be created inside the support structure between the bottom backplate electrode and the substrate. The substrate can be grounded. In other words, parasitic capacitances tend to be created between the top backplate electrode, membrane and bottom backplate electrode in combination, inside the support structure, i.e., in portions of the MEMS device excluding the air gap between the membrane and the top backplate electrode, as well as the air gap between the membrane and the bottom backplate electrode.
Parasitic capacitances are usually unwanted capacitances interfering with capacitances between both the membrane and the top backplate electrode as well as between the membrane and the bottom backplate electrode. Hence, capacitance values, intended to be transformed into electrical signals responsive to the movement of the membrane are interfered. In case the MEMS device is embodied as a double backplate microphone, for example, parasitic capacitances may influence the MEMS device such that the (electrical) output does not correspond to a correct reproduction of the (audible) input. While not mentioned, further sources of parasitic capacitances are conceivable.